The Golden Temple in the Indian city of Amritsar is a rare place of genuine calm in a chaotic, crowded country. Yet just under 30 years ago this complex of glittering shrines in Punjab was a scene of horrific violence when it was stormed by Indian security forces.

The temple had been occupied by militants under the command of a seminary student turned extremist, Jarnail Singh Bhindranwale, who called for an independent homeland for the Sikh minority of predominantly Hindu India.

Many hundreds, possibly thousands, were killed in the assault in June 1984. They included Bhindranwale himself as well as Sikh and Hindu pilgrims caught in the crossfire. The temple was badly damaged.

The botched operation, in which 136 Indian soldiers also died, led to the assassination of the prime minister, Indira Gandhi, by her Sikh bodyguards, which in turn provoked massacres of Sikhs. About 3,000 were stabbed, burned or beaten to death by mobs in Delhi, the capital, alone.

Nor did the violence stop there. Gandhi's Operation Blue Star horrified moderate Sikhs and in its aftermath, Punjab, the north-western state that is their historic homeland, was plunged into a violent insurgency that lasted almost a decade.

"The operation changed everything. So much of what unfolded in India afterwards can be traced back to Blue Star," said Hartosh Bal Singh, a political journalist in Delhi.

"We are shattered and numbed. We never believed the British government were helping in suppressing the Sikh movement. We thought it was the USSR who might have helped. We are very disappointed," said Kanwar Pal Singh, of the once-proscribed Dal Khalsa radical Sikh organisation.

Singh said he was writing to the British high commission in Delhi to call for a full statement from David Cameron.

The possibility that British special forces were consulted four months before the assault raised important new questions, said Singh, the journalist.

The difficulties encountered by the Indian security forces as they advanced into the temple complex have long been attributed to limited intelligence on the numbers, defences and firepower of the militants. The high civilian death toll has been blamed on the speed with which the operation was planned once negotiations with the extremists failed.

"Most Sikhs, even in India, will concede that [the operation] was necessary but badly botched. We thought it was all done in a hurry. But if they were thinking about it in February what were they doing over the following four months?" Singh said.

Opposition politicians in India also highlighted the new claim. The Congress party, in power at the time of the Blue Star operation and the riots that followed, has been leading a coalition government in India since 2004.

Gandhi and her son, Rajiv, who succeeded his mother as prime minister and was frequently criticised for failing to protect Sikhs in the days after her assassination, are iconic figures for Congress loyalists. The late Rajiv's son, Rahul, is the party's likely prime ministerial candidate in an election due within months; his mother, Sonia, is Congress party president.

Arun Jaitley, of the opposition Bharatiya Janata party, wrote that the new information "only lends credence to the fact that the government of India neither believed in nipping the problem at the initial stage nor in exploring alternative methods of evacuating the extremists from the Golden Temple". The perceived martyrdom of militants and civilians during Operation Blue Star remains a rallying call for Sikh hardliners in India and for many in the large diaspora in the UK and the US.

The general, Kuldip Singh Brar, said on Tuesday that allegations that the British government secretly helped Gandhi plan the Amritsar mission were fictitious. "All the plans [for Operation Blue Star] were laid and executed by Indian military commanders," he said.

"There was no question of getting help from the British government and no suggestion or mention at any stage of a British officer who had come and advised. It was a last-minute operation because the prime minister was negotiating with the Sikh leaders to arrive at an amicable solution. As a last resort, she ordered the operation."

In India, a plan to build a new shrine close to the temple dedicated to "the defenders, not the invaders" during the 1984 assault has caused fierce debate.

A spokesman for India's external affairs ministry said the Indian government had no information other than news reports about the claims but would be seeking further information from British counterparts.